Hydropneumatic devices for pressure intensifying are used for the force-loaded movement of a die for various applications, in particular for joining by shearing and upsetting. Devices of this type have a working plunger which is moved hydraulically in a state by an intensifier plunger which is actuated pneumatically.
Up to the point at which the intensifier plunger becomes active, the working plunger is moved pneumatically with a small force. In order to position the switchover point between a non-intensified and intensified movement of the working plunger at the location, at which a great force is required at the working plunger, a valve is used which applies a working pressure to the intensifier plunger in a manner which is dependent on a pressure in a pneumatic return stroke space of the working plunger for the pneumatic return of the working plunger counter to a working stroke direction. The valve operates according to the back pressure process in relation to a differential plunger. The differential plunger has two plungers which are connected to one another and of which one has a greater plunger diameter than the other. That side with the greater plunger diameter of the differential plunger is connected to a return stroke space of the working plunger. The side with the smaller plunger diameter is connected structurally to a pressure source which provides the working pressure and is regularly used to actuate the intensifier plunger when the valve switches through. A waste air throttle with a non-return function is provided in a connecting line between the return stroke space of the working plunger and the valve, by way of which waste air throttle the waste air speed and therefore the switchover time of the valve can be set.
In a basic position, the return stroke space is loaded with pressure, as a result of which the valve is situated in a position, in which the intensifier plunger is pressureless. In the case of a switchover to a working stroke, the working plunger is loaded with a pneumatic working pressure (fast stroke), with the result that the working plunger moves in a working stroke direction.
If the movement of the working plunger is decelerated by an increased counterforce or comes to a standstill, the pressure drops in the return stroke space, it being possible for the speed of the pressure drop on the larger side of the valve slide to be set by adjustment of the waste air throttle. If the pressure drops in front of the greater plunger diameter, this leads to the pressure at the smaller plunger diameter displacing the differential plunger and the valve switching. During this operation, a working pressure is switched through to a pressure space of the intensifier plunger, with the result that a movement of the working plunger then takes place with a great force in accordance with the transmission ratio of the intensifier plunger (power stroke). By way of the valve, it therefore always becomes possible, when the working plunger meets a counterforce and decelerates or sets its movement, to switch over to power stroke, in order to continue and to complete a desired working stroke with a considerably higher force.
The switching on and off of the air pressure can also be effected via externally actuated switching valves.
FIG. 2 shows a hydropneumatic pressure intensifier which is known from the prior art and has a convoluted travel of plunger elements only by way of example.
The following text with respect to FIG. 2 also applies in principle, however, to a pressure intensifier with a non-convoluted travel.
The pressure intensifier 1 comprises a pneumatically moved intensifier plunger 2 (called plunger in the following text), with a sealed plunger section 3 which is arranged displaceably in a pneumatic space (8a) (power stroke space) of a housing section 8 of the pressure intensifier 1. FIG. 2 shows the completely retracted position of the plunger 2, in which position pressure intensifying to a working plunger 4 has already taken place. The working plunger 4 is accommodated displaceably in a housing section 5 which is arranged in parallel.
In the stage which is shown, a plunger rod 2a of the plunger 2 is dipped into a high pressure hydraulic space 7 which is sealed by the plunger rod 2a via a seal (not shown). The high pressure hydraulic space 7 extends via a connecting line 7a into a hydraulic space section 7b in the housing section 5. The plunger is moved by pressure loading of the power stroke space 8a. 
The power stroke space 8a is sealed via a wall 9 and seals (not shown) to the plunger rod 2a of the plunger 2 with respect to a further pneumatic space 12.
The pneumatic space 12 is defined firstly by the wall 9 and secondly by an accumulator plunger 13. The accumulator plunger 13 has sealing elements (not shown) which firstly seal the accumulator plunger 13 toward the plunger rod 2a of the plunger which runs through the accumulator plunger 13, and secondly ensure separation of the pneumatic space 12 from a low pressure hydraulic space 18.
In the completely retracted state of the plunger 2, hydraulic liquid can be pressed out of the low pressure hydraulic space 18 into the high pressure hydraulic space 7 by way of a pneumatic movement of the accumulator plunger 13, since the plunger rod 2a is then pulled out of the high pressure hydraulic space 7 to such an extent that an opening 6a through the seal is exposed.
By way of hydraulic liquid flowing into the high pressure hydraulic space 7, the working plunger 4 is displaced in the working direction 5 (see arrow 19).
The supply can take place at a comparatively high speed and is called fast stroke.
The working plunger 4 has a plunger section 4a which is sealed with respect to the high pressure space 7 or 7b and a plunger section 4b which lies opposite the former in the working direction (arrow 19). A hydraulic liquid volume is enclosed in a hydraulic space 20 between the plunger sections 4a and 4b. 
The hydraulic space 20 is divided into a first region 21 and a second region 22 by a sealing section with respect to a plunger section 4c. A movement of the working plunger 4 can therefore take place only when the hydraulic fluid can flow over from the first region 21 and the second region 22 and vice versa. To this end, a regulating block (not shown) can be provided.
A movement sequence can be as follows: In a starting situation, the plunger 2 is retracted completely in FIG. 2 to the left-hand edge wall 8b of the power stroke space 8a. Via the accumulator plunger 13 which can be actuated pneumatically by compressed air loading of the pneumatic space 12, first of all hydraulic liquid is displaced from the low pressure hydraulic space 18 into the high pressure hydraulic space 7. As a result, a comparatively rapid stroke of the working plunger 4 can be brought about (fast stroke) by hydraulic fluid flowing over via the connecting line 7a into the hydraulic space section 7b. 
To this end, the hydraulic block (not shown) permits, for example, a corresponding rapid equalization of hydraulic liquid from the second region 22 into the first region 21.
In this phase, the working plunger 4 is under low pressure.
From a predefined displacement travel of the working plunger 4, the latter is to be loaded with high pressure. To this end, the plunger rod 2a of the plunger 2 enters into the high pressure hydraulic space 7 through the opening 6a by way of pneumatic loading of the power stroke space 8a. This operation is initiated (see below) by a valve for controlling the power stroke (power stroke valve). As a result of the ratio of the active cross sections of the plunger section 3 with respect to the plunger rod 2a, an enormous pressure intensification takes place into the hydraulic liquid in the high-pressure hydraulic space 7, with the result that the working plunger can be extended further with great force by way of the hydraulic liquid which is under high pressure, depending on how far the plunger dips into the high pressure hydraulic space 7 (power stroke).
It is also necessary in this movement that hydraulic liquid can flow over from the second region 22 into the first region 21 of the hydraulic space 20.
For a return movement of the working plunger 4, the regulating block (not shown) can be configured in such a way that a largely free flow of hydraulic liquid from the first region 21 into the second region 22 is possible. For the return movement, a pneumatic space 25 (return stroke space) is loaded with compressed air and, in the same way, the plunger 2 is moved back pneumatically over the pneumatic space 8, with the result that hydraulic liquid from the high pressure hydraulic space 7 can flow back into the low pressure hydraulic space 18 as a result of pressure loading in the pneumatic space 25.
As a result, the accumulator plunger 13 is also moved in the direction of the wall 9.
FIG. 3 shows a valve 26 which is known from the prior art for controlling the power stroke (power stroke valve) with a displaceably mounted differential plunger 27 with a plunger 28 with a large plunger face 29 and a plunger 30 with a small plunger face 31.
The plunger 28 moves in a pressure space 32 and the plunger 30 moves in a pressure space 33. FIG. 3 shows the position of the differential plunger 27, in which it is displaced as far as possible to the right.
In this position, there is a connection of an outlet 34, to which the power stroke space is connected, through the power stroke valve 26 to an outlet 35, via which air can escape via a baffle 36. The pressure space 33 is connected via an inlet 37 to a power stroke line (not shown). The function of the power stroke valve will be explained in the following text with reference to FIG. 4. Here, the connection to a pressure intensifier is shown in a very diagrammatic manner, for example to a pressure intensifier according to FIG. 2.
A changeover from fast stroke to power stroke takes place automatically when, during the fast stroke, the working plunger 4 encounters resistance at any desired point of the stroke and comes to a standstill. The side of the differential plunger 27 with the plunger 28 with a larger plunger face is connected via a pneumatic connection 39 at the connector 38 and a waste air throttle 40 to the return stroke space 25 of the pressure intensifier 1. The side of the differential plunger with the plunger 30 with the smaller plunger face 31 is connected via the connector 37 to a fast stroke line 41 of the pressure intensifier 1.
In the basic position of the pressure intensifier, the working plunger 4 and the plunger 2 and the accumulator plunger 13 are situated in a return stroke position, in which a return stroke pressure prevails in the return stroke space 25 via a return stroke line 42, which return stroke pressure loads the large plunger face 29 with pressure via the connection 39, the waste air throttle 40, the inlet 38 and the pressure space 32 and displaces the differential plunger 27 in the opposite direction to the pressure space 32.
In the case of a switchover into a fast stroke, the fast stroke pressure prevails at the small plunger face 31 of the plunger 30 via the fast stroke line 41, the inlet 37 and the pressure space 33. The working plunger then moves in the direction of the arrow 19 (see also FIG. 4).
The air which is enclosed in the return stroke space 25 cannot escape quickly enough via the return stroke line 42, with the result that there is a correspondingly high pressure in the pressure space 32 via the pneumatic connection 39 and the inlet 38, as a result of which the differential plunger 27 remains in the position which is shown in FIG. 3 despite a pressure in the pressure space 33, in which position the power stroke space 8a is still pressureless. If, however, the working plunger 4 encounters resistance and comes to a standstill, the pressure in the pressure space 32 drops via the waste air throttle 40, with the result that the power stroke valve 26 switches, by the differential plunger 27 moving into the pressure space 32 to such an extent that a connection of the inlet 37 to the outlet 34 takes place, as a result of which the power stroke space 8a is loaded with the fast stroke pressure or working pressure. The power stroke begins at this moment. The switchover time can be regulated via the waste air throttle 40, depending on how fast the enclosed air in the pressure space 32 can escape. If the pressure intensifier 1 is switched into the return stroke, the air escapes immediately from the fast stroke side of the power stroke valve 26 and the air which flows in onto the larger plunger face 29 brings about a switchover of the power stroke valve 26 substantially without delay back into the basic position.
The waste air throttle can also be replaced by a pneumatic switching valve for any desired switching through of the power stroke.